1. Field of the Invention
The invention relates to a method for measuring thicknesses of thin layers by means of X-ray fluorescence, in which a specimen having the layer to be studied is positioned in view and subsequently X-radiation is directed onto the layer being studied and emitted fluorescent radiation is detected by means of a detector and the layer thickness determined, as well as an apparatus for layer thickness is measured by X-ray fluorescence using an X-ray tube, a detector and an observing device with a focussing element.
Such methods and apparatuses are used for measuring the thickness and also the composition of X-ray fluorescence-producing layers, particularly galvanic layers in the micrometer range. The composition of alloy layers can also be determined.
2. Description of the Prior Art
In the case of the known apparatuses and methods, it is necessary for the surface of the workpiece to have a fixed distance from the detector and the X-ray source (both being fixed together). If it is necessary to study workpieces having different heights or thicknesses, then the distance between the specimen table and the detector or radiation source must be modified, so that the work surface facing the radiation source and the detector always has a constant distance from the radiation source. This can be brought about in that the workpiece-carrying specimen table or the measuring head containing the detector and radiation source is measured in the Z-direction, i.e. towards the X-radiation. The nominal distance is checked with optical means. This e.g. takes place in that the measuring head or specimen table are moved until the work surface is in the focus of a camera, i.e. is seen in focus by an optics.
Apparatuses for measuring the thickness by X-ray fluorescence are also known, which are able to operate with several fixed measurement distances. For specific applications shorter measurement distances are desired, because the measurement sensitivity for thinner layers is then better and in the case of a fixed collimator there is a smaller measuring spot due to the divergence of the primary X-ray bundle. With longer measurement distances the measurement sensitivity for thicker layers is better and in addition it is possible to measure depressions in the work surface or coating.
The tracing or tracking of a measuring head and specimen table involves a high mechanical cost and effort, because it is necessary to have a precise movement and positioning of a weight of a few kg. Thus, movement can also only take place slowly, so that it requires a considerable time, which is comparable to or longer than the measuring time.
Therefore the problem solved by the invention is to provide a method and an apparatus, in which the constructional expenditure for the precise movement of a specimen table or measuring head is obviated and there is no need for a time-consuming positioning of the work carrier or measuring head.